1. Field of the Invention
The present invention relates to a semiconductor element, and particularly to a field-effect transistor, of which gain coefficient β can be modulated in an analog manner as well as a logic circuit provided with the same.
2. Description of the Background Art
Semiconductor devices except for some special semiconductor elements have employed tri-electrode structures over about thirty years since these were invented. In a conventional semiconductor element, characteristics (gain coefficient β) depend on geometrical sizes set in a design stage, and therefore cannot be changed in a later stage. Thus, the gain coefficient β, which is a kind of unique characteristics of the semiconductor element, is fixed. Therefore, variations in characteristics due to variations in size of elements result in disadvantageous lowering of yields.
For overcoming the above, International Application Publication No. WO02/059979 has disclosed a semiconductor element, of which gain coefficient β can be modulated or determined independently of other semiconductor elements. The semiconductor element disclosed in the above International Application Publication No. WO02/059979 is provided with a control gate in addition to an ordinary MOS (Metal Oxide Semiconductor) gate. In accordance with a voltage applied to this control gate, a direction of an electric field in a channel direction is modulated to change effective gate length and gate width, and therefore, the gain coefficient β in the semiconductor element (field-effect transistor) can be modulated.
In the semiconductor element disclosed in the above International Application Publication No. WO02/059979, however, the control gate is arranged to form a certain angle θ with respect to the ordinary MOS gate for modulating the direction of the electric field at the channel. Accordingly, either the ordinary gate or the control gate does not have a geometry, which is not defined by a group including only straight and perpendicular lines, but is defined by a line group additionally including an oblique line(s) obliquely crossing the straight and perpendicular lines.
In particular, the above angle θ exerts a significant influence as a β-modulation parameter of the semiconductor element so that the oblique-line structure must be prepared with a high dimensional accuracy. This results in difficulty in gate preparation, and thus results in increases in fabrication cost and unstable production quality. More specifically, the above difficulty causes variations in characteristics of the fabricated semiconductor elements.